The present invention relates generally to the thermal spraying of powdered materials, as well as their application to surfaces as protective coatings.
A variety of thermal spray coatings have long been used to protect various components. A principal variety of thermal spray coatings to which the subject matter of the present invention pertains includes plasma sprayed coatings, although the improvements of the present invention will also pertain to other coatings and processes such as high velocity oxy-fuel (HVOF), having similar uses and properties. Plasma spray processes have been used to apply many different types of coatings to a variety of substrates, and find utility in numerous industries. One such application is responsive to conditions where a high degree of stress and wear is prevalent. In such a case, protective coatings containing carbides are often used. For example, the mid-span stiffeners used in the fan blades of aircraft gas turbine engines are commonly coated with a highly wear resistant tungsten carbide-cobalt (WC--Co) coating.
Popular techniques for the application of such coatings would include plasma spraying and high velocity oxy-fuel spraying. In the implementation of such coatings, hardness is often a factor of primary concern. However, both plasma sprayed coatings and high velocity oxy-fuel coatings have proven to be limited in their ability to meet the minimum mechanical property requirements for certain applications, particularly the requirements for minimum hardness.
In the practice of such processes, the current trend in the industry has been to develop processes that increase particle velocity and deposition rates. For example, an article by M. L. Thorpe and H. J. Richter, entitled "A Pragmatic Analysis and Comparison of the HVOF Process", Proceedings of the International Thermal Spray Conference & Exposition, Orlando, Fla. (May 28 to Jun. 5, 1992), at pages 137-147, discusses the increase of particle velocities in thermal spray processes. However, these known techniques have remained somewhat limited in terms of the overall hardnesses which could be achieved.
U.S. Pat. No. 5,082,179 (Simm et al.), U.S. Pat. No. 4,741,286 (Itoh et al.) and U.S. Pat. No. 4,236,059 (McComas et al.) disclose flame sprayed coatings applied using a plasma spray process. Simm et al. disclose relatively low velocity, low temperature spray methods, which are generally not suited to severe wear and erosion applications. Itoh et al. similarly disclose coatings applied with a low rate plasma gas flow. Generally speaking, the disclosed systems provide no specific guidance relative to the development of plasma sprayed coatings of a hardness commensurate with many present applications.
U.S. Pat. No. 5,330,798 (Browning) discloses flame sprayed coatings applied with a high velocity oxy-fuel spray process. The disclosed process uses the kinetic energy of impacting particles to obtain dense coatings. However, again, the resulting coatings are not of a hardness commensurate with many present applications.